Conservation and variation of nucleotide sequences within related bacterial genomes: enterobacteria

We have assessed the degree of relatedness of several portions of the Escherichia coli genome to the corresponding portions of the genomes of representative enteric bacteria, using the Southern transfer and hybridization technique (E. Southern, J. Mol. Biol. 98:503-517, 1975). The degree of relatedness varied among the regions examined. Judging both by the relative amounts of deoxyribonucleic acid in the various enteric genomes that are highly homologous and by the conservation of positions of restriction enzyme cleavage sites in these regions, the enteric genomes have diverged to greater extents in some parts of the genomes than in others. Portions of the genomes (including the tnaA and thyA genes, the trp operon, and one other unassigned segment) appear to have evolved in concert with the genome as a whole. By contrast, the lacZ gene and portions of the genome that are homologous to phage lambda vary more widely, perhaps reflecting a separate evolutionary origin for these segments of deoxyribonucleic acid.